1. Field of the Invention
Aspects of the present invention relate to a thin film transistor (TFT), a method of fabricating the same, and an organic light emitting diode (OLED) display device having the TFT, and more particularly, to a TFT, a method of fabricating the TFT in which an amorphous silicon (a-Si) layer is crystallized by controlling the amount of metal catalyst so as to not adversely affect the device characteristics of the TFT, and an OLED display device having the TFT.
2. Description of the Related Art
In general, a polycrystalline silicon (poly-Si) layer is widely used as a semiconductor layer for a thin film transistor (TFT) because the poly-Si has high field-effect mobility, can be applied to a high-speed operating circuit, and can be used to configure a complementary metal-oxide-semiconductor (CMOS) circuit. A TFT using the poly-Si layer is typically used as an active device of an active-matrix liquid crystal display (AMLCD) or a switching device or a driving device of an organic light emitting diode (OLED).
Methods of crystallizing an a-Si layer into a poly-Si layer may include a solid phase crystallization (SPC) method, an excimer laser crystallization (ELC) method, a metal induced crystallization (MIC) method, and a metal induced lateral crystallization (MILC) method. In the SPC method, an a-Si layer is annealed for several to several tens of hours at temperatures below the temperature of 700° C., a temperature at which a glass substrate used in a TFT for a display device may be deformed. In the ELC method, excimer laser beams are irradiated on an a-Si layer so that the a-Si layer is partially heated to a high temperature in a very short amount of time. In the MIC method, a metal, such as nickel (Ni), palladium (Pd), gold (Au), or aluminum (Al), is brought into contact with or doped into an a-Si layer to induce a phase change of the a-Si layer into a poly-Si layer. In the MILC method, silicide formed by a reaction of metal with silicon laterally diffuses so as to sequentially induce crystallization of an a-Si layer.
However, the SPC method takes too much time and may lead to deformation of a substrate because the substrate is annealed at such a high temperature for a long time. Also, the ELC method requires expensive laser apparatuses and results in formation of protrusions on the resultant poly-Si surface, thereby degrading interfacial characteristics between a semiconductor layer and a gate insulating layer therein. Furthermore, when the MIC or MILC method is employed, a large amount of metal catalyst may remain in a crystallized poly-Si layer, thereby increasing leakage current of a semiconductor layer of a TFT.
A vast amount of research has been conducted on methods of crystallizing an a-Si layer using a metal catalyst because the a-Si layer can be crystallized at a lower temperature for a shorter amount of time than in the SPC method. Typical methods of crystallizing an a-Si layer using a metal catalyst are the MIC method and the MILC method. In these methods, however, the device characteristics of a TFT may be degraded due to contamination caused by the metal catalyst.
In order to prevent the contamination caused by the metal catalyst, a method of fabricating a poly-Si layer by a crystallization method using a capping layer has been disclosed in Korean Patent Publication No. 2003-0060403. In this method, an a-Si layer and a capping layer are deposited on a substrate, and a metal catalyst layer is formed thereon. The substrate is annealed using thermal treatment or a laser so that the metal catalyst is diffused through the capping layer to the a-Si layer to form seeds. Thus, the a-Si layer is crystallized into a poly-Si layer using the seeds. In the above-described method, since the metal catalyst is diffused through the capping layer, excessive metal contamination may be inhibited. However, a large amount of metal catalyst still remains in the poly-Si layer. Therefore, it is necessary to find methods of fabricating poly-Si layers using the smallest amount of metal catalyst.